import re
regex = r"a\.b\([^()]*\)(?=;)"
s = ("a.b();\n"
"a.b(param1=1);\n"
"a.b(param1=1, param2=2);")
result = re.sub(regex, r"\g<0>.c()", s)
if result:
print (result)
aW1wb3J0IHJlCgpyZWdleCA9IHIiYVwuYlwoW14oKV0qXCkoPz07KSIKCnMgPSAoImEuYigpO1xuIgoJImEuYihwYXJhbTE9MSk7XG4iCgkiYS5iKHBhcmFtMT0xLCBwYXJhbTI9Mik7IikKCnJlc3VsdCA9IHJlLnN1YihyZWdleCwgciJcZzwwPi5jKCkiLCBzKQoKaWYgcmVzdWx0OgogICAgcHJpbnQgKHJlc3VsdCk=